1. Field of the Invention
This invention relates to a resin composition having excellent appearance and coating performances.
2. Related Art
Resin compositions prepared by graft copolymerizing a rubbery polymer with a vinyl cyanide compound and an aromatic vinyl compound, for example, an ABS resin, are widely used because of their excellent processability and mechanical strengths. However, when a polymer of a diene compound is used as the rubbery polymer of a resin composition, this type of resin composition is characterized with being insufficient in weather resistance, and therefore a product of the resin composition must sometimes be coated. Further, it is often coated for designing reasons.
If a molded resin article is coated with a coating material, small cracks can be formed on the surface of the molded resin article when it is contacted with the coating material or thinner, particularly when the coating material and the thinner are not appropriate or the conditions of coating work is not appropriate, for the article to be coated. Such cracks can absorb the coating material and, even if absorption of the coating material does not take place, the gloss vividness of the coating surface is deteriorated. When a coated resin article having such an inferior appearance is used next to a coated metallic article as in the case of interior and exterior coated parts of automobiles and motorcycles, the commercial value of the whole article is deteriorated.
It has been determined that the defective appearance of coated resin articles is attributable to the residual strain remaining in the molded resin. Accordingly, this phenomenon can be eliminated by annealing the molded article before it is coated. However, the annealing of large-sized articles requires large equipment, and increases the number of working steps which greatly deteriorates the productivity. Although coating materials and thinners which do not permeating the molded resins may be usable, their use is generally undesirable because a coating film produced therefrom is low in adhesive strength.
An attempt to solve the problem of defective appearance of coated resin article by the technique of improving the resin composition, can be referred to in Japanese Patent Application Kokai (Laid-Open) No. 54-94547. According to this technique, there is provided a resin composition prepared by compounding a rubbery polymer/vinyl cyanide compound/aromatic vinyl compound copolymer wherein the content of vinyl cyanide compound which is chemically grafted onto a rubbery polymer is 32 to 37% by weight, with a vinyl cyanide compound/aromatic vinyl compound copolymer wherein the content of vinyl cyanide compound is 20 to 37% by weight; and the content of vinyl cyanide compound in the total free polymer is adjusted to 22 to 35% by weight. In this technique, the content of the vinyl cyanide compound graft is made higher than the conventional content (25 to 30% by weight other than rubbery polymer component) of the vinyl cyanide compound in the components of general ABS resin, and thereby a high chemical resistance is given to the resin composition, the stress crack at the time of coating is prevented, and the absorption of the coating material is avoided. However, this technique wherein only the grafted chain is taken into consideration is disadvantageous in that the impact resistance of the resin composition is impractically deteriorated if the content of the vinyl cyanide compound is increased in order to additionally improve the coating performances.
In Japanese Patent Publication No. 63-30953, there is disclosed a technique which comprises using a general ABS resin in mixture with a vinyl cyanide compound/aromatic vinyl compound copolymer in which the vinyl cyanide compound content is as high as 38 to 65% by weight. According to this technique, however, the impact resistance property of the resin composition and its flow property during processing are impractically deteriorated if the content of vinyl cyanide compound is additionally increased in order to improve the coating performances.
In other words, none of the prior techniques has been capable of providing an ABS resin which exhibits good impact resistance and flow property during processing in addition to providing satisfactory coating performances.